Although large strides have been made in the recombinant expression of proteins, the efficient expression of certain classes of proteins remains a challenge. These include the small, highly stable pharmacologically active polypeptides with a high density of disulfide crosslinks. A major group within this general class includes the polypeptides present in animal venoms. Although several different phylogenetic lineages have evolved venoms independently, all polypeptides found have convergently evolved a common set of properties that allow them to be exceptionally stable upon injection into another organism. These polypeptides are of increasing interest because many of them have novel pharmacological activity and therefore serve as useful ligands in basic research or have direct diagnostic and therapeutic applications. One of these peptides, MVIIA a 25 amino acid peptide with three disulfide bonds, has become an approved drug for intractable pain.
When recombinant expression of small disulfide-rich polypeptides is attempted, the yields are generally low. A fundamental problem is that when expression levels are high, the resulting high concentrations of polypeptide in the cell lead to the formation of intermolecular aggregates, and recombinant polypeptides are mostly found in inclusion bodies. The ability to recover the polypeptide from an inclusion body in a biologically active form is not predictable and requires additional steps that vary depending on the polypeptide expressed.